The present invention relates generally to a system for detecting an electrical power line, and more specifically to an improved detector and method for use on board an aircraft for providing an indication of the time-to-impact with the power line if the aircraft remains on course.
Other devices have been proposed in the past for detecting the distance of a power transmission line or cable. Such devices generally fall within two categories, one being those that detect power lines by sensing the magnetic field associated with a conductor having an alternating current (AC) flowing therethrough, and the other type of device sensing the electric field associated with such power lines. From electromagnetic field theory, it is well known that an infinitely long conductor carrying a current will have an associated magnetic field H in a circular pattern concentric about the conductor, and an electric field E directed radially away from the conductor. From this, it is apparent that the means for sensing an electric and a magnetic field would be different, as well as the circuitry for interpreting the sensed electric and magnetic fields.
A system and method for detecting the direction of power lines from a helicopter by detecting an alternating magnetic field associated with the power lines is disclosed in U.S. Pat. No. 4,362,992 to Young et al. The magnetic field is detected by two vertical loop antennas, with each antenna defining a planar area and receiving a horizontal component of the magnetic field. The antennas are preferably positioned perpendicular to one another. A signal processor is provided to receive and interpret the magnetic field which is sensed by the vertical pair of antennas, and determines therefrom the direction of the power line emitting the magnetic field relative to the helicopter.
The Young et al power line detector also includes a third loop antenna which defines a horizontal planar area and detects the vertical component of the magnetic field. The detected vertical component is used to determine the magnitude of the magnetic field and to provide an early warning of the presence of power lines when the magnitude exceeds a predetermined level.
The Young et al power line detector compensates for the extraneous magnetic fields from sources other than the power line. For example, the effects of any static magnetic fields surrounding the antennas are cancelled by using three airs of Helmholtz coils. A flux valve is used to cancel the earth's magnetic field, and a latitude selector or attitude data unit compensates for the latitude or attitude of the helicopter. Compensation for local variations in the earth's magnetic field is provided by a unit programmed with known data supplied from tables. Such a cumbersome scheme of multiple compensation inputs renders such a magnetic field detector highly susceptible to errors.
Thus, the Young et al detection system merely indicates the relative direction of a power line from a helicopter, and provides an early warning of the presence of power lines. It does not provide or even suggest such useful outputs as the distance or range of the helicopter from the power line, or the time remaining until the helicopter would impact the power line.
Another magnetometer device for use on board an aircraft to detect the position of a magnetic source, such as a submarine, relative to the aircraft is disclosed in U.S. Pat. No. 4,309,659 to Yoshii. As shown, four magnetometer units A, B, C and D are positioned at the aircraft nose, tail and wing tips, respectively. The magnetometer units measure components of the magnetic field in two and in three mutually perpendicular directions. These components, along with a gyroscope signal, are inputs provided to a signal processor.
Magnetometer systems, such as that of Yoshii, typically detect variations in the earth's magnetic field caused by the presence of large magnetic bodies, such as a submarine. From this variation, the position of the magnetic body is computed relative to the position of the magnetometer system. The signal processor computes the distance, direction, attitude and magnetic moment to locate the magnetic source being sought. Typically, aircraft equipped with magnetometers for determining the location of a magnetic source are flown with wings level, even while making turns, otherwise pitch and roll compensation must be included in the magnetometer system.
Other magnetic field detectors are disclosed in U.S. Pat. Nos. 2,996,663, 3,582,932; 3,909,704; and 3,983,475.
There are a variety of proximity alarms for warning a heavy equipment operator, such as a crane operator, that the boom of the crane is approaching an energized power line. One such device is disclosed in U.S. Pat. No. 3,745,549 to Jepperson et al, which detects the proximity of the power line by detecting the electrostatic field associated therewith. An antenna is mounted to the heavy equipment extremity, such as the boom of a crane, the forks of a forklift truck, or the ladder portion of a ladder truck. A switching and control circuit includes antenna sensitivity controls, test circuitry and alarm circuitry. The alarm may be either a light or an audible signal which merely warns the operator when the equipment extremity enters the electrostatic field associated with an energized power line.
Further useful information is not provided by the Jepperson et al device, such as the direction of the power line relative to the equipment extremity, the distance therefrom, or the time before an impact of the equipment extremity with the power line would occur. The Jepperson et al device also requires calibration. If the sensitivity is adjusted to screen out electrostatic fields from nearby extraneous sources, such as high voltage transmission lines, the danger exists that a lower voltage line in the proximity of the equipment may go undetected. Additionally, such sensitivity controls may become misadjusted due to their being accidentally bumped or to the vibration of the heavy equipment during operation.
A proximity detector for warning the operator of a backhoe that the backhoe bucket is approaching an underground conduit is disclosed in U.S. Pat. No. 3,907,136 to Christides et al. This device detects conduit or pipe by transmitting an oscillating electric signal in the region where the backhoe is working, with such transmitters preferably being located on the backhoe support or stabilizer pads. The transmitters induce a small current in any buried electrically conductive conduit in the region. The induced current is detected by sensors mounted within the bucket teeth. Such transmitters may consume a great deal of electrical power in generating the electric field-inducing current. Also, the range of such transmitters would be limited by their power consumption, and are apparently only useful for detecting power lines or conduits within the immediate vicinity of the transmitters.
Other examples of devices used on heavy equipment to detect the presence of power lines are disclosed in U.S. Pat. Nos. 2,615,969; 3,168,729; 3,833,898; 4,064,997; 4,649,375; 4,675,664; and 4,727,447.
Several types of portable detection devices for locating buried metallic pipes have been proposed, which sense an electromagnetic field emitted from the pipe. U.S. Pat. No. 3,988,663 to Slough et al detects the location and depth of buried metallic pipes which carry AC signals impressed thereon as a result of various industrial activities in the vicinity.
U.S. Pat. No. 3,889,179 to Cutler discloses a portable buried pipe locator. External excitation of the pipe by an external power supply is required to provide an emission source to generate an electric field which is detected by the locator. Such a locator first requires that an external location of the pipe be known and the connection be made. The depth of the buried pipe is computed by a triangulation method using multiple readings in the devices of both Slough et al and Cutler.
Another electromagnetic field detecting device for locating buried pipe is disclosed in U.S. Pat. No. 3,893,025 to Humphreys, which also requires external excitation of the buried pipe. The external excitation is provided by transmitters which impress radio frequency signals upon the buried cable or pipe. Two vertically displaced antennas are used to detect the emitted electric field. The difference in the electric field detected by each antenna and the fixed distance between the antennas are used to determine the depth of the pipe. Other devices for locating buried conductors are disclosed in U.S. Pat. Nos. 4,295,095 and 4,672,321.
Thus, a need exists for an improved apparatus for detecting the presence, range and direction relative to an aircraft of an electric power line, as well as for determining the time remaining for a pilot of the aircraft to make a course correction to avoid an impact with the power line.